Increasing demand for information transfer has made it desirable to employ the existing telephone network for this purpose. The telephone network makes available a variety of grades of services, and it is obviously desirable to employ the lowest grade, and therefore, the most inexpensive, service, as is consistent with acceptable bit error rates.
The typical communication medium made available by the telephone network is an analog channel; accordingly, the need to transfer information which is digital in form has lead to the development of devices to convert digital data into a signal that will propagate on an analog channel, and to correctly convert this signal back into digital form.
Devices of this sort have been available for some time, and are characterized as "modems", shorthand for modulator-demodulator. Stated in the simplest terms, the goal of the modem is to adapt the digital signals to the characteristics of the available transmission medium, i.e., the phone channel. Phone channels come in at least two different types, i.e., leased and switched. The leased channels have characteristics which are maintained to within specified limits. The switched channel, on the other hand, has properties which vary from channel to channel, and are usually characterized by their mean and standard deviation.
The most popular prior art modems can be characterized as high baud rate modems. Generally, these modems require rather precise time equalization to compensate for the telephone channels envelope delay distortion. This particular type of distortion, which comes about because the phase delay encountered by signals of different frequencies is different, is extremely troublesome. This is particularly so since the envelope delay distortion will vary for different lines in the switched network, and thus, the various parameters of the equalization networks cannot be standardized. Rather, these networks must adapt or "learn" proper parameters through actual use. Since most adaptive equalizers require special training signals, the learning process cannot take place during the transmission of real data. Accordingly, the duration of the training period reduces the time available for transmission of actual data. Since the physical channel actually used from call to call will be different in a switched network, the equalization may require adjustment on each different connection. See, for example, "Modems" by Davey, in Proc. IEEE, November 1972, pages 1284-92.
There have been suggestions in the prior art to implement modems which transmit a plurality of bits per signalling element or baud. With multi-bit bauds, the baud rate, which is the rate at which different bauds are transmitted, can be reduced, resulting in a lower baud rate than bit rate. Some examples are Ito, U.S. Pat. No. 3,349,182; Walker, U.S. Pat. No. 3,456,194; Hauber, U.S. Pat. No. 3,579,110; Low, U.S. Pat. No. 3,659,053; Walker, U.S. Pat. No. 3,431,143.
While these systems can reduce the baud rate for a given bit rate, they are not directed at using the switched telephone network as a communication channel, and therefore, do not address the problems caused by envelope delay distortion.
Several of the suggestions referred to above employ a plurality of sub-carriers, modulating each sub-carrier to increase the bit rate without increasing the baud rate. The suggestions also employ correlation detection, that is, each different sub-carrier is separately detected by use of a locally generated noise free replica of the sub-carrier.
In more detail, Whang (U.S. Pat. No. 3,524,023) indicates that a significant problem with respect to data transmission over the switched telephone network is the variation in line characteristics that are expected. One solution to the problem was equalization, i.e., an effort to "smooth" the line characteristics by compensating for distortions in the time domain. Unfortunately, the best equalizers take a significant amount of time to determine the proper equalization for each line. Since equalization is required on each different connection, the time required for equalization is a substantial disadvantage and efforts are going forward to reduce this time, see IEEE Transactions on Communications, Vol. COM-26 #5 (May 1978).
Whang, in U.S. Pat. No. 3,524,023, taught a different technique, i.e., instead of attempting to employ as much of the available bandwidth as possible, and running into severe equalization requirements, he suggested employing a relatively small portion of the available bandwidth, e.g., about 1/3. With this technique, variable equalization was not required since, in the chosen third of the bandwidth, all the telephone lines appear substantially similar. The disadvantage to the Whang approach is, of course, the two-thirds of the available bandwidth that are discarded. Accordingly, Whang claimed the capability of transmitting at a rate of 2400 bits per second through the switched telephone network, which at the time, was considered a radical advance.
Walker, in U.S. Pat. No. 3,456,194, took a different approach to data transmission although not related to switched networks as a communication media. Instead of discarding two-thirds of the available bandwidth, he attempted to employ as much of the available bandwidth as possible, and instead employed a gap or guard space in his regime. More particularly, to provide a large bit/baud ratio, a plurality of sub-carriers are each modulated. To allow coherent demodulation, the sub-carriers are harmonically related. The modulated sub-carriers are transmitted for a time termed the symbol time and modulation is effected at rate corresponding to the symbol time. Walker proposed coherent demodulation, and used a guard space, or gap to reduce intersymbol distortion. In effect, the demodulation process operated for less than the entire symbol time, the difference being the gap or guard space. However, since Walker chose the fundamental as the symbol rate, i.e., the symbol interval was equal to the period of the fundamental, the demodulation took place for an interval which was less than the period of the fundamental. Accordingly, Walker's demodulation took place with non-orthogonal signals, i.e, he could and did expect distortion caused by a contribution in one channel or sub-carrier, from other channels or sub-carriers. To attempt to overcome this difficulty, he attempted to precalculate the contribution in each channel, from all the other channels, and "compensate" for this contribution by using a weighting network. The difficulty with this technique is that the precalculations necessary to determine the weightings require knowledge of the characteristics of the transmission medium. Since Walker is disclosed in the context of a radiated transmission, the approach may be workable. However, employing the Walker technique in the switched telephone network does not appear advantageous since it is the very variations in the characteristics of the communication medium which made use of the switched telephone network difficult in the first place. Therefore, it would appear that applying the Walker technique to data transmission over the switched network would not be appropriate since the weightings will properly apply to only nominal lines which, in the context of the switched network, are usually not available.
However, Walker appears to have an advantage over Whang in that Walker merely discards a portion of the available time bandwidth product equal to the ratio of the gap time or guard space to the symbol time whereas Whang employs only about one-third of the available time bandwidth product.
In accordance with the teachings of the present invention, the disadvantages inherent in Walker are obviated while at the same time, retaining the advantage of employing much more of the time bandwidth product than used by Whang.
It is therefore one object of the invention to provide a transmission system capable of reliably transmitting 9600 bits per second over an unconditioned line in the switched telephone network. It is another object of the invention to perform the foregoing without requiring automatic or variable time equalization as in many prior art modems. It is another object of the invention to achieve the foregoing data transmission rates with acceptable bit error rates and with equipment of reasonable cost, size, etc. It is yet another object of the invention to achieve the foregoing and other objects by using a modem which is readily interfaced with conventional data terminal equipment.